The extract below might throw some light on the pyrite issue, it seems there was plenty of it at DeGrussa (see second sentence) and we all know about the economics of Sandfire's mine!

DeGrussa Processing Plant overview

By Ruth Nicholaidis, Sandfire Resources NL

The primary ore at DeGrussa is a volcanic-hosted massive sulphide (VHMS) deposit, which is a high grade copper deposit with payable gold and silver. The dominant sulphide minerals in the primary ore body are pyrite (35 per cent), chalcopyrite (14 per cent), pyrrhotite (4 per cent) and sphalerite (2.5 per cent). Non-sulphide gangue (NSG) minerals present in the ore include magnetite (2.5 per cent), stilpnomelane (2.5 per cent), minnesotaite (1 per cent) and talc (1 per cent) amongst others. Quite uniquely, the copper is only present as chalcopyrite in the primary ore (+99 per cent). The gold and silver are largely present as an electrum and are predominantly associated with the pyrite. Figure 3 shows typical ROM stocks.

Figure 3. NSG, iron and copper sulphides visible in the ROM stockpiles.

There are two main ore types at DCM which behave differently in the flotation circuit: pyrite dominant and pyrrhotite dominant ore. The pyrite dominant ore type is characterised by the presence of fine grained interstitial chalcopyrite intergrowths within pyrite. The fine grained ore microtextures limit the chalcopyrite liberation, and therefore the treatment of this ore achieves a lower copper recovery and lower concentrate grade. Finer grinding in the primary mills, and regrind circuit often improves flotation performance. For pyrrhotite dominant ore types, the chalcopyrite grain size is typically coarser, with less intergrowths of gangue minerals. The chalcopyrite has a higher liberation level in comparison to the pyrite ore types, and the copper recovery is higher, and a higher concentrate grade is more easily achieved.

Blending is critical for the operation with ROM stockpile grades varying from 1 per cent Cu to >15 per cent Cu. Ore is stockpiled separately primarily based on stope, which can be further separated based on different mineralogy of particular ore parcels within a stope. Blending aims to provide a consistent copper feed grade to the concentrator, as well as considering other drivers of metallurgical performance, penalty components that may report to the concentrate stream, ore hardness, magnetics content, etc.